1. Field of the Invention
The invention concerns an optical telecommunication system.
2. Description of the Prior Art
A system of this kind comprises optical conductors transmitting optical signals produced by one or more laser sources and amplitude modulated by the signal to be transmitted. This signal is usually a telephone signal or a data signal. The advantages of optical transmission are low signal distortion and low attenuation. However, there is some attenuation; its effects are always felt over long distances. This is why spaced repeaters are provided in optical telecommunication systems to amplify the naturally attenuated optical signal.
These repeaters include optical amplifiers. An optical amplifier is usually formed by a doped optical fiber, for example one doped with erbium ions Er.sup.+++, and by a pump laser for inverting the population of electrons in the fiber, this inversion producing the amplification effect.
If the input optical signal is polarized, it can be amplified anisotropically by the amplifier because the gain is lower in the direction of polarization of the signal than in the orthogonal direction. In other words, the amplifier is more strongly saturated in the signal polarization direction than in the orthogonal direction.
This differential gain degrades the signal to noise ratio, which affects transmission quality.
This property of optical amplifiers is a drawback of all optical telecommunication systems since in these latter systems the optical signal is produced by a laser supplying a polarized wave.
Various solutions to this problem have already been proposed.
For example, in French patent 2 717 331 there is disclosed a polarizer imposing a particular polarization direction and an electro-optical phase modulator receiving the signal polarized at 45.degree. to the crystal axes of the modulator. The control voltage applied to the crystal periodically modifies its refractive index. This index variation, which is not the same for the two crystal axes, is effected at a period which is less than the saturation time of the amplifier. It produces a phase shift between the components of the polarization vector on the two crystal axes. This phase shift causes rotation of the polarization vector and therefore "scrambling" of the polarization. This renders the gain of the amplifier isotropic.
In another example (U.S. Pat. No. 5,361,270), the polarization is scrambled by two phase modulators in series.
The invention aims to simplify the implementation of the polarization scrambling.